The present invention relates to a surface acoustic wave filter which is used for cellular phones and other mobile communication equipment.
FIG. 10 shows a circuit diagram of a conventional SAW (surface acoustic wave) filter. FIG. 10 shows SAW resonant elements 101, 102, and 103 which are disposed on a piezoelectric substrate, and input terminal 104 and output terminal 105. Here, three resonators configure a band-stop filter.
FIG. 11 shows characteristics of the band-stop filter configured by three resonators which are disposed on a piezoelectric substrate made of LiTaO3 (lithium tantalate) using conventional circuit structure. FIG. 12 shows an equivalent circuit diagram of the SAW resonant element shown in FIG. 10. In FIG. 12, reference number 111 is a parallel capacitor C0, reference number 112 an equivalent series inductor L1, reference number 113 an equivalent series capacitor C1, and reference number 114 an equivalent series resistor R1.
But the material property of the piezoelectric substrate shown in FIG. 12 mentioned above determines a physical constant, or capacitance ratio xcex7=C0/C1. A series resonance frequency Fs and a parallel resonance frequency Fp of a SAW resonator element (for LiTaO3, xcex7=about 13) are
FS=1/(2xcfx80(L1xc3x97C1)0.5)
Fp=1/(2xcfx80(L1xc3x97C1xc3x97C2/(C1+C2))0.5)
respectively. Therefore, because capacitors C1 and C2 share a relation determined by the capacitance ratio xcex7, a frequency separation between the series resonance frequency and the parallel resonance frequency of the SAW resonant element as electrical characteristics can not be set arbitrarily. As a result, the attenuation characteristics and the pass characteristicsxe2x80x94both are the important filter characteristicsxe2x80x94also have a limitation as shown in FIG. 11.
An object of the present invention is to attain a SAW filter which can set attenuation characteristics and pass characteristics arbitrarily.
In order to attain the object, the SAW filter of the present invention comprises comb-shaped electrodes facing each other disposed on the same piezoelectric substrate, at least two resonators with an input and output terminals, and an equivalent transmission line which is connected between an output terminal of the first resonator and an input terminal of a second resonator. The phase of the transmission line can be adjusted from 5 degrees to 175 degrees arbitrarily at the series resonance frequency of the resonators.
The band-stop filter or the band-pass filter based on the present invention can set the attenuation characteristics and the pass characteristics arbitrarily.